Inductors are used in radio frequency (“RF”) and microwave circuits in a wide variety of applications. A coil of fine wire is often used to generate inductance; however, physical inductors do not provide pure inductance. The coils of an inductor capacitively couple to each other, generating what is commonly known as parasitic capacitance, also known as stray capacitance. Stray capacitances also arise from other sources, such as the inductor coils coupling to and/or through other structures of the circuit in which the inductor is used. The combination of the inductance and parasitic capacitance forms an inductance-capacitance (“LC”) circuit having a resonant frequency, commonly called the self-resonant frequency.
Stray capacitance is undesirable for inductors that will be used in broad-band high-frequency applications, such as RF chokes used in bias tees, because it reduces the self-resonant frequency of the inductor. The series self-resonant frequency is the frequency at which the inductor appears as a short circuit. Conical coils, also referred to as conical inductors, are used to extend the operating range of RF chokes by increasing the series self-resonant frequency compared to a cylindrical coil having a diameter equal to the wide end of the conical inductor, while providing good low-frequency performance. Basically, the conical inductor acts like a series of progressively (physically) larger inductors, moving from the narrow end to the wide end. Conical inductors are further described in U.S. Pat. Nos. 6,344,781 and 6,236,289.
Conventional microwave circuits use special holders to align and support a conical inductor on a substrate (see FIGS. 1A-1D, below). The holder is typically made of plastic and manually attached to the substrate using an adhesive, such as an epoxy adhesive. The substrate has fiducial marks that indicate where the operator should place the holder; however the manufacturing repeatability is relatively low. Then, the conical inductor is placed in the holder and secured with adhesive. The holder holds the conical inductor on its axis, but the conical inductor can slide back and forth in the holder, introducing another source of alignment/placement error. A way to assemble conical inductors on substrates that avoids the disadvantages described above is desired.